One-component (1K) baking systems based on polyurethane are heat-curable materials, stable on storage at room temperature, for preparing paints, inks and adhesives. They consist in general of blocked polyisocyanates which in the course of thermal curing are consumed by reaction with hydroxyl-containing polyesters, polyacrylates, other hydroxy-functional polymers and/or mixtures of different polymers. Another possibility to obtain raw materials for baking enamels which are stable on storage at room temperature is the partial blocking of the isocyanate groups of polymers containing both blocked isocyanate groups and hydroxyl groups.
The principal compounds used to block polyisocyanates and 1K baking systems are ε-caprolactam, methyl ethyl ketoxime (butanone oxime), secondary amines and also triazole and pyrazole derivatives, as described for example in EP-A 0 576 952, EP-A 0 566 953, EP-A 0 159 117, U.S. Pat. No. 4,482,721, WO 97/12924 or EP-A 0 744 423. Malonate blocking is also possible. With this kind of blocking, however, the blocking agent is not cleaved back; instead, a transesterification reaction takes place on the diethyl malonate radical.
Depending on the blocking agent used, temperatures of 100–160° C. are employed in producing coatings from the 1K PU baking systems. The selection of the appropriate blocking agent for the particular system, however, is made not only according to the baking temperature. Other factors, such as yellowing tendency, odour and storage stability of the systems, for example, also play an important part. Since especially in recent times a concern has been to minimize the baking temperature of coating systems, it is necessary in each case to find a compromise in terms of the composition of the coating materials and the properties of the coating. From this it is evident that there is a need for new baking systems which have optimum performance properties even at relatively low baking temperatures.
In the past already a large number of experiments have been undertaken aimed at lowering the baking temperature of 1K systems through the use of catalysts. Thus in EP-A 0 761 705, for example, organic bismuth compounds are claimed for the catalysis of partly or fully blocked polyisocyanates. U.S. Pat. No. 5,859,165 describes reaction products of manganese, cobalt, nickel, copper, zinc, germanium, antimony or bismuth and/or their oxides as catalysts for blocked poly(thio)isocyanates. EP-A 0 726 284 describes in general terms metal salts and/or metal complexes for catalysing the reaction of blocked polyisocyanates with polyols, although the examples disclose only dibutyltin dilaurate and dibutyltin acetate specifically.
In order to reduce the use of organic solvents and hence to reduce the emission of these solvents into the environment, and in order to improve working conditions on the coating line through reduced solvent emission, recent years have seen the development of 1K coating systems comprising water as a predominant solvent component. An overview of this technology is given by D. A. Wicks and Z. W. Wicks in Progress in Organic Coatings 2001, 41(1–3), 1–83. This technology is spreading. The presence of the aqueous solvent and/or dispersion medium imposes different requirements regarding the use of catalysts than is the case with what are termed solvent-borne systems. Thus in the latter systems, when using catalysts, there is no need to ensure that the catalyst used is stable to water or to hydrolysis. Consequently, the common catalysts employed in solvent-borne 1K systems cannot generally be used in what are termed aqueous systems. Known representatives of such catalysts, which possess a high activity (i.e. a marked reduction in the baking temperatures) include, for example, bismuth 2-ethylhexanoate and organic tin(IV) compounds such as dibutyltin dilaurate (DBTL). Besides these a range of further compounds have been disclosed, described in the above-cited article by Wicks et al. It is also known that bismuth carboxylates are hydrolysed in water.
To date only a few catalysts have been disclosed for accelerating the curing of aqueous one-component systems. WO 95/04093 outlines organotin-based systems. These are catalysts which are used in particular in systems for electrocoating, where curing normally takes place at high temperatures of approximately 170° C. or more. The blocking agents and polyisocyanates used in each case are not specified in the examples. However, owing to ecological considerations, the use of organotin catalysts is not desirable. The activity of these and other catalysts in comparison to other catalyst systems is also described in the following application.
The Description of WO 00/47642, page 4, cites very specific examples of catalysts for 1K aqueous applications. Thus organotin compounds and lead compounds are described whose use in coatings, however, is not desirable, from standpoints of ecology.
WO 00/47642 also contains a reference to a catalyst for aqueous one-component systems which is based on the reaction of bismuth oxide with a carboxylic acid having a carbon chain length of from C11 to C36. Although hydrolysis of the catalyst takes place with this system as well, the catalyst is said to reform from the constituents at the relatively high baking temperatures of more than 165° C. up to 180° C. and to possess a high catalytic activity. The use of this catalyst system, however, is tied to very specific resins and/or alcohol components.
The activity of the catalyst system described is described only for specific resins—in this case, cationically hydrophilicized resins, i.e. resins obtained by reacting, for example, an expoxy resin containing bisphenyl A with an amine. Depending on the amine used (primary, secondary, tertiary) and in the presence of an excess of the epoxy resin and in the presence of water and neutralizing acid it is also possible for quaternary ammonium groups to form. Hence the resin is in principle amine-containing, which is unsuitable for the development of an automotive surfacer that is intended to have low yellowing and good long-term stability.
As an alternative to cationic hydrophilicization it would be possible to prepare an aqueous 1K PU system by adding surface-active substances or emulsifiers. The catalyst system presented therein is not described for such a coating system of this kind.
Also possible is hydrophilicization with, for example, anionic hydrophilicizers (e.g. by carboxylic acids), or nonionic hydrophilicizers such as, for example, by polyethers (incorporated into the resin and not as an individual constituent, as in the case of the emulsifiers) for the preparation of an aqueous 1K system. The catalyst system presented therein, however, has likewise not been described for such a coating system.
On the basis of the different possibilities of hydrophilicizing 1K systems (cationically, by emulsifiers, by anionic or nonionic hydrophilicization) the use and activity of the catalyst system described in WO 00/47642 in systems other than cationically hydrophilicized systems is not obvious. For example, cationic hydrophilicizing can act through ammonium salts as a ligand for stabilization. This stabilizing effect is absent in the 1K systems, which are not cationically hydrophilicized.
Moreover, the aforementioned publication describes only alcohol-blocked isocyanates. A typical blocking agent for blocking the isocyanate exclusively described therein, polymeric) MDI (methylene-phenyl diisocyanate), is butoxyethoxyethanol (butyl carbitol). In addition, 2-ethoxyethanol and 2-methoxyethanol are also cited. The elimination of this blocking agent (in actual fact a urethane cleavage) re,quires high temperatures: baking is carried out at temperatures of 165–180° C. over a period of 20 minutes.
For the intended use as coating composition for passenger cars it is desirable to find catalysts which allow a one-component system to be cured at temperatures of not more than 140° C., and preferably at an even lower temperature. Accordingly, no catalyst is known at present whose use in aqueous systems based on a broad spectrum of blocking agents, blocked (poly)isocyanates and hydrophilicizing methods would allow the baking temperatures to be lowered to the desired level.
The object was therefore to find a catalyst suitable for general use which is effective at low baking temperatures and with a multiplicity of blocking agents and resins and hydrophilicizing agents. Account ought at the same time to be taken of ecological aspects.